Current dual chamber, multi-mode, cardiac pacemakers typically employ atrial and ventricular endocardial pacing leads having one or two distally located pace/sense electrodes that are adapted to be attached in the right atrium and right ventricle, respectively, and operate to sense the atrial and ventricular electrogram (EGM) and deliver pacing pulses to each chamber, depending on the operating mode. Dual chamber demand cardiac pacing is dependent upon the retention of the atrial pace/sense electrode(s) at the atrial site. The dislocation of the atrial pace/sense electrode(s) to a location inferior to the AV node can result in the loss of sensing of the atrial EGM events (principally the P-wave). However, the R-wave of the ventricular EGM as well as other electrical signal peaks of the QRST complex may be readily detected if there is good electrode-tissue contact or may be intermittently detected if the electrode-tissue contact is intermittent. The relatively high gain setting of the atrial sense amplifier necessary to sense the relatively low amplitude P-wave may also contribute to mistaken "sensing" of peaks of the QRST complex and other spurious signals as P-waves. As a result, atrial and ventricular pacing may be inhibited (which may not be undesirable under the circumstances) or become erratic, which could cause a ventricular pacing pulse to be delivered at an undesirable time in the pacing cycle, possibly provoking a tachyarrhythmia. At the least, the benefits of synchronous dual chamber pacing in sustaining a physiologic heart rate and adequate cardiac output would be lost. In addition, atrial pacing pulses may in some cases trigger ventricular depolarizations, possibly inducing ventricular tachyarrhythmias. The same sort of problem may in some cases also occur in response to implant of an atrial electrode low in the right atrium, which may also result in atrial pacing pulses triggering ventricular depolarizations, particularly where higher amplitude pacing pulses are employed, as may be the case during atrial antitachycardia pacing.
In proposed dual chamber PCD systems having the capability of detecting and treating atrial arrhythmias with at least a limited menu of anti-tachyarrhythmia therapies, also referred to as supraventricular arrhythmias and including atrial fibrillation and atrial flutter, the correct diagnosis of the nature of a detected tachyarrhythmia so that an appropriate treatment can be delivered is crucial. Typically, in proposed dual chamber PCD systems, at least both atrial and ventricular pacing and sensing functions are provided in conjunction with tachyarrhythmia detection and anti-tachyarrhythmia therapy delivery in at least one of the chambers. Such dual chamber PCD systems may only provide atrial anti-tachycardia pacing therapies of the types described below or may include atrial cardioversion/defibrillation capabilities as further described below. The failure to deliver the appropriate therapy or the delivery of an inappropriate therapy to treat an apparent atrial tachyarrhythmia can progress to or trigger more serious ventricular tachyarrhythmia. Consequently, a great deal of effort has been undertaken to refine the diagnosis of the tachyarrhythmia and to define the appropriate therapy in response to the diagnosis.
One approach to detection of dislocation or inappropriate location of atrial leads is set forth in U.S. Pat. No. 5,713,932, issued to Gillberg et al. and incorporated herein by reference in its entirety, which discloses a dual chamber cardiac stimulator in which the determination of the occurrence of a dislocation of the atrial pace/sense electrode is effected by applying a test pace pulse to the atrial pace/sense electrode; detecting the immediately following ventricular depolarization from a ventricular sense electrode; measuring the interval between the delivered atrial pace pulse and the detected ventricular depolarization; comparing the measured interval to a threshold AV interval; and determining that the atrial pace/sense electrode is in contact with the right atrium if the measured interval is longer than the threshold AV interval. As disclosed, the determination is effected by: providing a first signal when the measured AV interval exceeds the threshold AV interval and a second signal when the measured AV interval is less than the threshold AV interval; applying a series of M atrial pace pulses to the atrial pace/sense electrode; counting the number of first and second signals provided in response to the series of atrial pace pulses; and determining that the atrial pace/sense electrode is located in the right atrium when a predetermined number of first signals are provided in a series of delivered atrial pace pulses.